Kenwood TH-D74E Service Manual page 13

FM tuner circuit
The FM broadcasting band (76MHz~108MHz) signal can be received using an SMA antenna.
The reception signal passes through the band-pass filter and enters the RF amplifier (Q675) for the amplification of the signal, and
the signal is applied to the FM tuner IC (IC674). The signal is demodulated in the FM tuner IC to generate an audio signal and
de-emphasized. Then the signal is applied to the main MPU/DSP (IC702) for loudness control. Then the signal enters the AF
amplifier (IC880) for the last amplification of the signal, passes through the SP MUTE switches (Q881 and Q883), and drives the SP.
SP / MIC SP SW
J880 Q881,883
SP GND
SP SW
EXT SP
INT SP
2.1.7.2 Transmission AF circuit
Modulation signal circuit
The transmission AF signal from the microphone is amplified by the MIC amplifier (IC881), applied to the main MPU/DSP (IC702)
through the CODEC IC (IC707), and pre-emphasized.
Each signal made in the main MPU/DSP (IC702) is processed as shown below. The TONE and DTMF signals are pre-emphasized
here. Packet transmission data at 1200 bps is subjected to the baseband filter and sub-carrier modulation. Packet transmission data
at 9600 bps is subjected to the baseband filter only.
The transmission AF signal that has been level adjusted to an appropriate value passes through the CODEC IC (IC707), and then
branched into two. One of them is amplified by the MOD VCO AMP (IC720) and applied as a high-speed modulation signal (MOD) to
the D/A converter (IC671). Then the signal is applied to the VCO B. The other is amplified by the MOD TCXO AMP (IC720) and applied
to the reference oscillator (X362).
VOX circuit
The transmission AF signal is amplified by the VOX amplifier (IC881) and detected by the DC-detector (D882) to obtain a DC voltage
in proportion to the input signal level. This voltage is input into the main MPU/DSP (IC702) to monitor the audio signal level.
MIC AMP
SP / MIC
IC881
J880
EXT MIC
INT MIC
2.1.8 Control system
The control circuit consists of the main MPU/DSP (IC702), its peripheral circuits, and sub MPU (IC661). The IC702 has the following
primary roles.
(1) Switching of transmission and reception by the PTT signal.
(2) Reading of the system program from the memory.
(3) Control of the audio mute circuit according to decode data.
The IC661 has the following primary roles.
(1) Frequency program transfer to the PLL and IF IC control.
2.1.8.1 MAIN MPU
The main microprocessor with digital signal processor (MPU/DSP) (IC702) incorporates a 32-bit RISC processor, fixed- and floating-
point VLIW digital signal processors, and peripheral functions.
This microprocessor operates at a 139.2 MHz clock rate and 3.3/1.8/1.2 VDC. The microprocessor controls the transmission and
reception of data to and from the flash memory, mobile DDR, control circuit, display circuit, sub MPU, and external devices.
AF PA
CODEC IC
IC880
IC707
6.8V_AVR
IC882
AF VOLUME
AFBS
Q885
SPMI
DC SW
IC702
Fig.10 Reception AF circuit
CODEC IC
IC707
VOX DET
D882
VOX
PRE EMPHASIS AGC
TONE
PRE EMPHASIS
DTMF
LPF SUB CARRIER
1200sps
DATA
LPF
AGC
9600sps
DATA
IC702
Fig.11 Transmission AF circuit
IC674
FM TUNER
nd
ADC 2 MIXER LNA(AGC)
DE EMPHASIS
BEEP, DTMF
MOD AMP
IC720
AGC
AGC
AFB_FM
AFA
(From TXRX PCB)
RF AMP BPF(VCAP)
Q675 D673/674 D672
FM Tuner
Din
RF SW
(From
TXRX PCB)
BPFR /EANTS
SQA
SQB
SMA
SMB
(From TXRX PCB)
DAC
IC671
VCO_MOD
(To TXRX PCB)
TCXO_VCON
(To TXRX PCB)
Freq
(From TXRX PCB)
(No.RA044<Rev.001>)1-13